The weight of Xiaoming and Xiaohua plus the weight of Obana Xiaoqing, minus the weight of Xiaoming and Xiaoqing, equals twice the weight of Xiaohua, so the weight of Xiaohua is (52+57-55) ÷ 2 = 27 (kg).
Xiaoming's weight is 52-27=25 (kg).
Xiaoqing weighs 57-27=30 (kg).